The present invention relates to a process for scrambling digital video data as well as to a process for descrambling digital video data scrambled according to the scrambling process of the invention.
The invention applies more particularly to conditional access systems for which the video data are formatted according to the MPEG2 video standard and conveyed according to the MPEG2 System transport standard.
By way of non-limiting example, a conditional access system such as the one mentioned above can be a pay television system broadcasting scrambled programmes.
As is known to the person skilled in the art, within the context of a conditional access system operating according to the MPEG2 standard, the video data are scrambled in such a way that, after scrambling, they no longer comply with the MPEG2 video standard.
In the case in which the user receiving the scrambled programme has acquired the entitlement for descrambling the programme, the scrambled video data are descrambled so as to be restored to the MPEG2 video standard. The user""s MPEG2 decoder is then able to decode the descrambled video data which it receives.
In the case in which the user has not acquired the entitlement to descramble the scrambled programme, the video data are not descramble. The user""s MPEG2 decoder is then totally unable to recognize the video data. No image appears on the screen: the screen is black.
The complete absence of images prevents even a glimpse of the broadcast programme and, consequently, makes it impossible to attract the attention of any potential future subscribers to the provider distributing the programmes.
The invention does not have this drawback.
Thus, the invention relates to a process for scrambling video data in the MPEG2 video format, the video data representing at least one block of at least one image coded in INTRA mode and comprising data (AC, DC) arising from a xe2x80x9cdiscrete cosine transformxe2x80x9d operation and consisting, for each block, of a first coefficient (DC) representing the mean intensity of the pixels of the block and of at least one second coefficient (AC) representing the intensity variations between pixels of the block. The scrambling of the video data is carried out by an operation for processing the data (AC, DC) arising from the xe2x80x9cdiscrete cosine transformxe2x80x9d operation compatible with the MPEG2 video standard.
The invention also relates to a process for descrambling scrambled video data, the descrambling of the video data being performed under the action of a descrambling key. The video data are data scrambled according to the abovementioned scrambling process of the invention and the descrambling key consists of at least one of the data arising from the operation for processing the data arising from the xe2x80x9cdiscrete cosine transformxe2x80x9d operation.
The invention also relates to a device for scrambling video data in the MPEG2 video format, the video data representing at least one block of at least one image coded in INTRA mode (I) and comprising data (AC, DC) arising from a xe2x80x9cdiscrete cosine transformxe2x80x9d operation and consisting, for each block, of a first coefficient (DC) representing the mean intensity of the pixels of the block and of at least one second coefficient (AC) representing the intensity variations between pixels of the block, characterized in that it comprises means for carrying out the scrambling of the video data by an operation for processing the data (AC, DC) arising from the xe2x80x9cdiscrete cosine transformxe2x80x9d operation compatible with the MPEG2 video standard.
The invention further relates to a device for descrambling video data scrambled according to the abovementioned scrambling process of the invention, the descrambling of the video data being performed under the action of a descrambling key, characterized in that it comprises means for descrambling the video data under the action of a descrambling key consisting of at least one of the data arising from the operation for processing the data arising from the xe2x80x9cdiscrete cosine transformxe2x80x9d operation.
The invention further relates to a decoder of video data coded according to the MPEG2 video standard, comprising a device for descrambling scrambled video data, characterized in that the descrambling device is a descrambling device such as the abovementioned device according to the invention.
An advantage of the invention is that it implements a process for scrambling and a process for descrambling video data which are compatible with the MPEG2 video standard.